Summary 1.Using data from a 20-year study of individually marked red-billed choughs, we examine how reproductive performance varies with age in male and female breeders, and investigate whether population-level trends result from changes in individual performance and/or the phenotypic composition of the breeding population. 2. Across the population, mean clutch size, the probability of breeding successfully and the number of offspring fledged during successful attempts increased and then declined with female age. Male age did not explain a significant proportion of the residual variation. 3. All three measures of reproductive performance improved and then declined with age within individual females. 4. Females that died young laid relatively small clutches and fledged few offspring before death. Thus mean performance improved across young age classes partly because some poor breeders were absent from older age classes. 5. Females that ultimately reached the greatest ages had laid small clutches and fledged few offspring during their first few breeding attempts. Females that were more productive when they were young had relatively shorter lives. These data indicate a trade-off between early reproduction and future survival in choughs, and suggest that individuals that reach old age are phenotypically distinct from an early stage in their breeding lives. 6. We emphasize that age-specific changes in mean reproductive performance observed across wild populations are due to a complex interplay between improvement and senescence at the individual level, as well as changes in the phenotypic composition of the breeding population.

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Summary1. The consequences of environmental variability for life-history evolution are predicted to depend on the pattern of covariation amongst life-history traits. Using data from a 20-year study of individually marked red-billed choughs, we investigate the short-and long-term life-history consequences of population-wide variation in reproductive conditions, and demonstrate clear among-cohort variation and covariation in life-history parameters. 2. The mean number of offspring fledging per breeding event varied among years, and was correlated with environmental conditions (temperature and rainfall) during the months preceding breeding. As the variance in breeding performance did not differ among years and choughs did not miss breeding seasons, variation in environmental conditions affected the whole breeding population. Thus the quality of the chough's breeding environment varied amongst years. 3. Juvenile survival, the probability of recruitment to the breeding population and breeding longevity varied amongst cohorts, and these were positively correlated with the quality of the cohort's natal environment. Offspring fledging under good conditions were more likely to survive to breeding age and recruit, and had longer breeding lives than offspring fledging under poor conditions. 4. Age at first breeding varied amongst cohorts, and increased with population size at maturity rather than natal conditions. 5. The total number of offspring that recruits ultimately fledged varied primarily with breeding longevity rather than recruitment age. Thus, the consistent positive covariation amongst life-history traits meant that the total number of offspring fledged by recruits during their breeding life varied amongst cohorts, and was correlated with the quality of a cohort's natal conditions. Choughs fledging under good conditions themselves ultimately fledged more offspring. 6. Such environmentally determined variation in offspring fitness is expected to influence optimal patterns of parental investment. We discuss the predictions that environmental variability should select for investment in adult survival and for reduced reproductive effort in poor years.

Summary 1.Identifying which age-specific demographic rates underlie variation in a population's growth rate ( λ ) is an important step towards understanding the population's dynamics. Using data from a 20-year study of marked individuals, we describe patterns of demographic variation and covariation in the Scottish red-billed chough population ( Pyrrhocorax pyrrhocorax ), and investigate which demographic rates have the greatest projected and realized influence on λ . 2. Survival, the probability of breeding and breeding success varied with age in this population. Data were sufficient to estimate year-specific probabilities of first-year, second-year and adult (all ages over 2) survival and mean breeding success. A population trajectory modelled using these parameter estimates closely matched census data, suggesting that estimates and simplifying assumptions were sufficient to accurately describe important demographic processes. 3. Elasticity analyses based on stage-classes for which year-specific survival was estimable suggested that λ was more elastic to variation in adult survival than first-or second-year survival or breeding success. These ranks were consistent across all 15 years for which λ could be estimated directly, although the elasticity of adult survival declined with population growth. 4. Survival and breeding success were positively correlated across years. λ remained most sensitive to adult survival when this demographic covariation was incorporated into elasticity analyses. 5. However, elasticities calculated from a fully age-structured model suggested that λ was more elastic to variation in first-and second-year survival than to survival at any individual older age class. These ranks were robust to realistic demographic variation, but sensitive to postulated patterns of demographic covariation. We emphasize that covariation should be measured and incorporated into elasticity analyses, and that estimated elasticities must be interpreted in the context of the way in which stage-classes are defined. 6. Of the demographic rates in which we quantified between-year variation, first-year survival varied most, followed by second-year survival, breeding success and adult survival. These rates consequently contributed more equally to variation in λ than elasticities predicted. Overall, variation in λ was caused primarily by variation in survival rather than breeding success, and variation in prebreeding survival accounted for 56% of the total variation in λ .

Summary 1.Understanding the degree to which reproductive success varies with an individual's age and lifespan, and the degree to which population-level variation mirrors individual-level variation, is central to understanding life-history evolution and the dynamics of age-structured populations. We quantified variation in the survival probability of offspring, one key component of reproductive success and fitness, in relation to parent age and lifespan in a wild population of red-billed choughs (Pyrrhocorax pyrrhocorax). 2. On average across the study population, the first-year survival probability of offspring decreased with increasing parent age and lifespan; offspring of old parents were less likely to survive than offspring of young parents, and offspring of long-lived parents were less likely to survive than offspring of short-lived parents. 3. However, survival did not vary with parent age across offspring produced by groups of parents that ultimately had similar lifespans. 4. Rather, across offspring produced by young parents, offspring survival decreased with increasing parent lifespan; parents that ultimately had long lifespans produced offspring that survived poorly, even when these parents were breeding at young ages. 5. The average decrease in offspring survival with increasing parent age observed across the population therefore reflected the gradual disappearance of short-lived parents that produced offspring that survived well, not age-specific variation in offspring survival within individual parents. 6. The negative correlation between offspring survival and maternal lifespan was strongest when environmental conditions meant that offspring survival was low across the population. 7. These data suggest an environment-dependent trade-off between parent and offspring survival, show consistent individual variation in the resolution of this trade-off that is set early in a parent's life, and demonstrate that such structured life-history variation can generate spurious evidence of senescence in key fitness components when measured across a population.